1. Field of the Invention
The present invention relates to an emitter follower circuit by use of a bipolar transistor.
2. Description of the Related Art
An emitter follower circuit is generally used to decrease output impedance and to shift the level of a signal. The emitter follower circuit is often formed in a semiconductor integrated circuit, and is generally connected between signal processing circuits which are formed in the semiconductor integrated circuit or to an output terminal of the signal processing circuit.
The circuit shown in FIG. 1 is conventionally used as the emitter follower circuit. Referring to FIG. 1, the base of an NPN-type bipolar transistor (to be referred to as an NPN transistor, hereinafter) 501 is connected to an input terminal (IN), and the emitter of the NPN transistor 501 and one of ends of a current source 503 which outputs current of a specific current value is connected to an output terminal (OUT). A power supply terminal (VCC) is connected to the collector of the NPN transistor 501 and a power supply terminal (VEE) is connected to the other end of the current source 503.
As another conventional example, a filter circuit is described in Japanese Laid Open Patent Application (JP-A-Heisei 7-183763). In this reference, bipolar transistors 14 and 22 of an emitter follower circuit are connected to transistors 24 and 26, respectively. The transistors 24 and 26 are supplied with a current generated by a temperature characteristic compensating current generating circuit 30. The temperature characteristic compensating current generating circuit 30 is structured to have the same temperature characteristic as that of output impedance of the transistor 14 or 22. Thus, temperature compensation is performed.
As seen from FIG. 1, a signal is outputted from the output terminal (OUT) to have a signal level lower than a signal level at the input terminal (IN) by the forward direction voltage between the base and the emitter VBE of the NPN transistor 1. That is, in the emitter follower circuit, the signal having a signal level shifted by the forward direction base-emitter voltage VBE from the signal level at the input terminal (IN) is outputted from the output terminal (OUT).
It is known that the forward direction base-emitter voltage VBE of the NPN transistor 1 shown in FIG. 1 has a predetermined temperature dependent drift, and the value of the temperature dependent drift is about -2 mV/.degree. C. with respect to temperature increase.
Now, in FIG. 1, it is supposed that the power supply terminals VCC and VEE are supplied with voltages of 3 V and 0 V, respectively. Also, it is supposed that a signal having the signal level of 3 V is supplied to the input terminal (IN), and a signal having the signal level of 2.1 V is outputted from the output terminal (OUT) when the junction temperature of the emitter follower circuit is 0.degree. C. In case where the junction temperature rises from 0.degree. C. to 100.degree. C., the forward direction base-emitter voltage VBE of the NPN transistor 1 becomes smaller by 0.2 V because of the influence of the above-mentioned temperature dependent drift of the forward direction base-emitter voltage VBE. Thus, the signal outputted from the output terminal (OUT) has the signal level of 2.3 V. That is, there is a problem in that the level shifted voltage with high precision cannot be obtained to the temperature change in the conventional emitter follower circuit shown in FIG. 1.
By the way, the forward direction base-emitter voltage VBE of the NPN transistor 501 necessarily varies due to the change of manufacture condition in the manufacturing process of the NPN transistor 501. It is known that the variance of the forward direction base-emitter voltage VBE has no predetermined tendency and the forward direction base-emitter voltage VBE varies at random. For this reason, in the conventional emitter follower circuit shown in FIG. 1, there is another problem in that the level of the signal outputted from the output terminal (OUT) changes at random due to the change of the forward direction base-emitter voltage VBE so that a level shifted voltage with high precision cannot be obtained.